1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly, to a zoom lens system which is preferably used for a video camera, digital camera, film camera and broadcasting camera, etc.
2. Description of the Related Art
With miniaturization of a single-lens reflex camera, lens shutter camera, digital still camera, video camera, etc., there is a demand for a short-length and compact zoom lens as an image-taking lens used for these cameras in recent years.
For example, Patent document 1 discloses a zoom lens made up of four lens units of positive, negative, positive, and positive refractive power, respectively with each lens unit consisting of a few lenses. This zoom lens realizes compactness and a zoom ratio (magnification-varying ratio) of ×3 or more by arranging two or more aspherical surfaces from the third lens unit onward, etc.
Furthermore, as disclosed in Patent document 2, a compact and wide-angle zoom lens having a zoom ratio of up to approximately ×3.5 is known, which consists of four lens units; positive, negative, positive and positive refractive power, realized by limiting the number of lens components of each lens unit and specifying refractive power distribution.
Patent Document 1, Japanese Patent Application Laid-Open No. H8(1996)-220439 (corresponding to: U.S. Pat. No. 5,768,029)
Patent Document 2, Japanese Patent Application Laid-Open No. H9(1997)-184982 (corresponding to: U.S. Pat. No. 5,859,729)
With regard to a zoom lens in general, it is necessary to set the lens structure of each lens unit in order to reduce the size of the entire lens system and obtain good optical performance over the entire zoom range while maintaining a desired zoom ratio. In a zoom lens, increasing the refractive power of each lens unit reduces the amount of movement of each lens unit during zooming, making it possible to shorten the total length of the zoom lens. However, simply increasing the refractive power of each lens unit will also increase a variation of aberration caused by zooming, causing a problem that it will be difficult to correct this aberration appropriately.
For many zoom lenses, it is important to make the structure of the optical system as simple as possible while maintaining high optical performance.
In the zoom lens disclosed in Patent document 1, all the four lens units are each constructed of three lenses or less and the total number of lenses is small, which makes the lens system compact. On the other hand, this zoom lens uses two or more aspherical lenses with strong refractive power made of a glass material whose manufacturing is relatively difficult. This results in a tendency to make manufacturing difficult.
The zoom lens disclosed in Patent document 2 achieves a zoom lens with four lens units having a structure with fewer lenses, but when the zoom ratio exceeds ×3, it is still necessary to use two aspherical lenses. This results in a tendency to make manufacturing difficult. Furthermore, since the first lens unit having the largest aperture in the optical system has relatively strong positive refractive power, it has a lens shape with strong curvature, which results in a tendency to make machining of lenses difficult.
In this way, in a zoom lens consisting of four lens units of positive, negative, positive and positive refractive power, in order to realize a small lens system with a high zoom ratio and fewer lenses, it is necessary to set conditions of movement of each lens unit caused by zooming and refractive power, etc., of each lens unit appropriately. Unless these are satisfied, various types of aberration will occur increasingly and it will be more difficult to obtain images of good quality over the entire zoom range.